Can I modify and retransmit a declaration that is already sent?

Yes, you can. However, you must use the out_request_no of the original request for the retransmission, and the retransmission must be at least 5 minutes later than the original request. You can modify the following fields for the retransmission:
a) merchant_customs_code
b) merchant_customs_name
c) customs_place
d) amount
e) sub_out_biz_no